1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and more particularly, to a sheet processing apparatus and an image forming apparatus in which a bookbinding process is performed by folding a sheet or a sheet stack on which images have been formed.
2. Description of the Related Art
Up to now, as an example of an image forming apparatus such as a copying machine and a laser beam printer, there is an image forming apparatus including a sheet processing apparatus that performs a book binding process in which discharged sheets having images formed thereon are taken in the sheet processing apparatus, the sheets are subjected to a stapling process in substantially middle portions of the sheets, and are then subjected to a process of folding the sheets in half and the like.
In the sheet processing apparatus, the sheets on which images have been formed in the image forming apparatus main body are first sequentially taken in the sheet processing apparatus, and then a stapler unit is driven to staple the substantially middle portion of the sheet stack. After that, the sheet stack is conveyed to folding means to fold the sheet stack in half by the folding means.
The folding means includes a folding roller pair and pushing means composed of a pushing plate and the like. In folding the sheet stack in half, a portion of the sheet stack corresponding to a staple position is pushed to a nip part between the folding roller pair by the pushing plate. Then, when the sheet stack is thus pushed to the nip part between the folding roller pair, the sheet stack is pressed by the folding roller pair and conveyed while being folded in half at the staple position in the middle of the sheet stack. The half-folded sheet stack is discharged to a discharge tray in a state where the sheet stack is bound.
One folding roller of the folding roller pair is set to be movable in a releasing direction by about the thickness of the sheet stack so as to nip the sheet stack. The movable folding roller is mounted to a swingably supported holding plate and is brought into press contact with the other folding roller whose position is fixed.
Here, as an example of press-contacting means for bringing one folding roller into press contact with the other folding roller, there is one in which a pressure contact force is generated using an urging member such as one linear spring (for example, see JP H11-322180 A).
However, in the sheet processing apparatus of this type, in a case where a friction coefficient between sheets is low when the folding process is performed, slippage of the sheets is caused when the half-folded sheet stack is drawn into the folding roller pair, which may cause a tear in the staple position of the sheet stack. Such the tear is likely to be caused immediately after the folding process is started. In this case, particularly when an image is formed on a portion at which the folding is started and to which toner is adhered, an inner side of the folded portion of the sheet is likely to be unfolded when the sheet is drawn into the folding roller pair, thereby easily causing the tear of the sheet.
In view of this, in order to avoid causing such the tear during the folding process, a margin is provided in advance to the middle portion of the sheet which corresponds to a folded portion of the sheet The margin is provided to the middle portion of the sheet other than the image forming portions of the sheet, thereby making it possible to make the friction coefficient between sheets larger in the folded portion, and fold the sheet stack without causing any tear or wrinkles.
However, in recent years, colorization has been progressed in the field of the image forming apparatus, and full-color images are formed on sheets in many cases, so it is necessary to reduce the margin provided to the folded portion. For this reason, a sheet stack is pushed by the pushing means to a nip point between the folding roller pair so as to fold the sheet on which an image has been formed at the folded portion without causing slippage of sheets.
In the conventional sheet processing apparatus of this type, in a case where the number of sheets to be nipped is small (for example, 2 sheets), a height of the folded sheets is constant even when the pressure contact force of the folding roller pair is small (for example, 160 N or more) On the other hand, when the number of sheets to be nipped is large (for example, 15 sheets), the height of the folded sheets is not constant if the pressure contact force of the folding roller pair is set to be large (for example, 700 N or more).
Here, up to now, the pressure contact force is set to only one value, so there arises the following problem. That is, for example when the pressure contact force is set in consideration of folding a plurality of sheets, a pressure contact force of 500 N is applied even in a case of folding 2 sheets though 2 sheets can normally be folded by a pressure contact force of about 160 N.